In the drilling of deep boreholes, the rotary drilling technique has become a commonly accepted practice. This technique involves using a drill string which consists of numerous sections of hollow pipe connected together and to the bottom end of which a drill bit is attached. By imparting axial forces onto the drilling bit and by rotating the drill string either from the surface or using a hydraulic motor attached to the drill string, a reasonably smooth and circular borehole is created. The rotation and compression of the drilling bit causes the formation being drilled to be crushed and pulverized. Drilling fluid is pumped down the hollow center of the drill string through nozzles on the drilling bit and then back to the surface around the annular space between the drill string and the borehole wall. This fluid circulation is used to transport the cuttings from the bottom of the borehole to the surface where they are filtered out and the drilling fluid is recirculated as desired. The flow of the drilling fluid also carries out other functions such as cooling and lubricating the drilling bit cutting surfaces and exerts a hydrostatic pressure against the borehole walls to help contain any entrapped gases that are encountered during the drilling process.
The need to measure certain parameters at the bottom of a borehole and provide this information to the driller has long been recognized. These parameters include, but are not limited to the temperature, pressure, inclination and direction of the borehole, and can include various geophysical and lithological measurements. The challenge of measuring these parameters in the hostile environment at the bottom of a borehole during the drilling process and conveying this information to the surface in a timely fashion has led to the development of many devices and practices.
It is an advantage to have the ability to send data from the bottom of a bore well to the surface, while drilling and without the use of wires or cables and without the repeated interruption of drilling activity. Tools that have the above ability are commonly referred to as “measurement while drilling” or “MWD” tools. Pressure pulses in the drilling fluid may be used to encode and transmit data to the surface of the earth from an MWD tool at the bottom of a borehole.
There are a variety of different measured parameters that may be transmitted to the driller. These range from the simplest measurement of the temperature at the bottom of the borehole to fully integrated products that provide a full range of measurements including but not limited to inclination, azimuth, toolface (rotational orientation of the drill string), pressures, temperatures, vibration levels, formation geophysical properties such as resistivity, porosity, permeability, density and insitu formation analysis for hydrocarbon content.
Due to the harsh nature of the downhole drilling environment, MWD tools necessarily have to be robust in design and execution. In addition, the constant flow of drilling fluid through or past the MWD tool causes significant erosion of exposed components and can cause significant damage to tools if improperly designed or operated.
It is understood that the term “drilling fluid” or “mud” is used here to represent an extremely wide variety of water or oil based liquids of varying densities, viscosities and contaminant content. The need to keep the borehole hydrostatic pressures high in order to contain or reduce the risk of a gas pocket from escaping the bore well results in the drilling fluid being weighted with additives to increase its density. These additives often tend to be abrasive in nature and further exasperate the erosion problems associated with the flow of the fluid past the tool.
In addition, the need to preserve and maintain the quality of the bore well and to prevent or reduce the risk of the bore well caving in, other filler materials are added to the drilling fluid to aid in bonding the bore well walls. These filler materials tend to be granular in nature and clog or cover inlet and outlet ports, screens and other associated hydraulic components that are part of most MWD tools.
Further, the extreme temperatures and pressures that are present in the bottom of the bore well often necessitate the use of expensive and exotic sealing mechanisms and materials, which increase the costs of operating the MWD tools, and thereby reduce their usability to the wider market place.
Still furthermore, due to the high costs associated with drilling oil and gas boreholes, any time that is spent repairing, maintaining or servicing failed or nonfunctional equipment results in a severe reduction in the productivity of the whole drilling operation. As such, MWD tools have always needed to be designed, built and operated with a need for high quality and reliability.
Thus, an important goal in the design of MWD tools is to provide a pulse generator which can operate reliably in the hostile environment produced by the exposure to drilling mud and other downhole conditions.